An important perception for ammunition manufacturers from warring conflicts in recent times consists in the fact that the weapons and ammunition platforms currently in use are only able to offer insufficient protection against enemy attacks. These new threat scenarios exist essentially in conditions of enemy fire on light and medium armoured vehicles, where the armour of the latter is relatively easily penetrated. The threat is also intensified by the fact that the weapons that provide the threat are easily transportable and are to be found in large numbers in uncontrolled circulation. There thus exists a definite need for improvement with regard to the ability to resist the detrimental mechanical agencies caused by bombardment of the ammunition with e.g. a hollow charge jet, hot metal fragments, or bullets. The vulnerability of ammunition is in fact a systems issue, but one in which the propellant charge powder exerts a strong influence.
Moreover the recent past has shown that the risk of conflicts in hot climate zones must be classified as clearly increasing. Such types of “out of area” actions in hot climate zones in general demand an improvement in the chemical stability of a propellant charge powder, such that its safety during handling, use and storage remains fully guaranteed. Further examples, where an improvement in chemical and thermal stability is required, occur in modern fighter aircraft in the form of extremely severe thermal cycling of the ammunition carried, with temperature peaks of more than 100° C. (“fast cook-off”), or in the ability of ammunition to resist fires (“slow cook-off”). The chemical stability of a propellant charge powder, which determines both its service life and also its “cook-off” temperature, thus represents a further area of activity with a need for improvement.
For several years, therefore, developments have been in progress with the objective of preparing propellant charge powders with a high performance potential and improved properties with regard to vulnerability (i.e. with regard to mechanical agencies) and “cook-off” (i.e. with regard to thermal agencies). Here there exists the challenge that a propellant charge powder to be used for military purposes must exhibit as high an energy density as possible, but at the same time should exhibit as low a vulnerability as possible with regard to mechanical and thermal agencies. This requirement is of outstanding importance for enclosed spaces such as for example occur in tanks, armoured troop carriers, or warships.
For quite some time an attempt has been made to fulfil this requirement by means of so-called “insensitive ammunition” (IM), for which purpose a new LOVA (low vulnerability ammunition) propellant charge powder has been developed. These propellant charge powders typically contain between 60 to 80% by weight of a crystalline explosives material, and about 10 to 25% by weight of an inert or energetic binder. Typical explosive materials in LOVA propellant charge powders are cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX). Present LOVA propellant charge powders consist typically of a synthetic inert or energetic elastomeric polymer binder in which the crystals of the explosive in question are embedded. Typical binders are CAB and HTPB (inert) and GAP, poly-AMMO and poly-BAMO.
In the case of propellant charge powders (in short: TLPs) for weapons applications a distinction is made between “homogeneous” and “composite” (heterogeneous) formulations, where homogeneous formulations include monobasic and dibasic propellant charge powders. In extensive IM tests it has been found that a LOVA propellant charge powder based on an inert binder exhibits advantages compared with conventional powders with regard to thermal agencies (cook-off). In contrast it has been shown that compositions of this type can detonate in the event of mechanical agencies, a fact that has up to the present time hindered their wide-scale introduction and use (c.f. e.g. L. M. Barrington, Australian Defence Force (ADE), DSTO-TR-0097).
An example of a LOVA-TLP with an energetic synthetic binder is described in U.S. Pat. No. 6,228,190, where the binder consists of a nitratoalkyl-substituted alkyl ether-prepolymer with reactive hydroxy end groups and a cross-linking agent on the basis of a polyvalent isocyanate compound. From practice it is known art that powders constituted from such types of binders are brittle at lower temperatures and that their manufacture is very expensive and difficult.
LOVA-TLPs with an elastomeric binder containing polyurethane represent a further class of LOVA-TLPs of known art and are described in U.S. Pat. No. 4,925,503, U.S. Pat. No. 4,923,536 and U.S. Pat. No. 5,468,312 amongst other sources. The extended chain polyurethane polyacetal elastomer binder is obtained by means of a reaction of a dihydroxy-terminated polyacetal-homopolymer with an alkylene-diisocyanatc, subsequent conversion of the resulting isocyanate-terminated prepolymer with a dihydroxy-terminated polyacetal copolymer and a final reaction of this clastomeric intermediate stage with an organic polyisocyanate. Since the manufacture of this elastomeric binder system requires a number of synthesis steps the costs are very high. In addition it has been shown in the past that reproducibility presents great problems such that the LOVA-TLPs obtained cannot be manufactured with the required uniformity of product properties. For these reasons LOVA-TLPs on this basis have not been able to achieve acceptance on a broad front up to the present time.
A further class of LOVA-TLP uses cellulose acetate or derivatives of this (e.g. cellulose acetate butyrate, CAB) as the elastomeric binder. Compositions of this type are described in U.S. Pat. No. 6,984,275 amongst other sources.
The LOVA compositions of known art are unsatisfactory, since their reproducibility is insufficiently guaranteed and the manufacturing costs are relatively high. They have therefore not found practical application.